This invention relates to a heat-sensitive recording material excellent in thermal response.
Heat-sensitive recording materials generally comprise a support and formed thereon a heat-sensitive recording layer containing an electron donative, normally colorless or light-colored dye precursor and an electron-accepting color developer. When heated with a thermal head, a thermal pen, a laser light, or the like, the dye precursor and the color developer react instantly to give a recording image. These heat-sensitive recording materials are disclosed, for example, in Japanese Patent Examined Publication Nos. 43-4160 and 45-14039. Since these heat-sensitive recording materials have advantages in that recording can be obtained by using a relatively simple apparatus, maintenance is easy and no noise is generated, these materials are widely used in the fields of measuring instrument recorders, facsimile machines, printers, computer terminals, labels, automatic ticket vending machines, etc. Particularly in the field of facsimile machines, a demand for heat-sensitive method increases remarkably and high-speed transmission is to be realized in order to reduce a transmitting cost. In response to speed-up of facsimile machines, a demand for more sensitive heat-sensitive recording materials increases. For example, since a high-speed facsimile machine can send and receive a message of a standard A4 size manuscript in several seconds to 20 seconds, an electric current passing through a thermal head of the facsimile machine is repetition of a very short time of several milliseconds or less and a heat energy generated thereby transmits to a heat-sensitive recording sheet to carry out an image formation reaction.
In order to carry out the image formation reaction by the thermal energy transmitted in such a short time, it is necessary to make heat-sensitive recording materials excellent in thermal response. In order to increase the thermal response, it is necessary to improve compatibility of the color developer with the dye precursor. For such a purpose, a sensitizer is used, if necessary. Since the sensitizer has a function to accelerate a color forming reaction by dissolving or involving the neighboring dye precursor and color developer when it melts by the transmitted heat energy, it is one method for making the sensitivity of heat-sensitive recording material higher to improve the thermal response of the sensitizer. Such a method can be attained by adding, for example, waxes (Japanese Patent Unexamined Publication No. 48-19231), nitrogen-containing compounds and carboxylic acid esters (Japanese Patent Unexamined Publication Nos. 49-34842, 50-149353, 52-106746, and 53-5636), naphthol derivatives (Japanese Patent Unexamined Publication Nos. 57-64593 and 58-87094), naphthoic acid derivatives (Japanese Patent Unexamined Publication Nos. 57-64592, 57-185187, 57-191089 and 58-110289), benzoic acid ester derivatives (Japanese Patent Unexamined Publication Nos. 57-148688, 57-182483, 58-112788, and 58-162379), para-benzylbiphenyl (Japanese Patent Unexamined Publication No. 60-122193), biphenyls (Japanese Patent Unexamined Publication No. 61-272189), diphenoxyethanes (Japanese Patent Unexamined Publication No. 60-56588) and sulfides (Japanese Patent Unexamined Publication No. 61-242884). But heat-sensitive recording materials produced by such methods are still insufficient in developed color density and optical density.